Even the best ones heave themselves across the finishing line, panting for breath with bloodied feet and wobbly limbs.

This is the state in which Barack Obama finds himself after five months of primaries.

His claim of victory feels historic - you can smell it in the humid summer air.

The man who was born around a time when blacks had to sit at the back of the bus has become the first African-American who could sit in the Oval Office.

Mrs Clinton has been on a winning streak even while losing overall

It was a night to remember, but beyond the rhetoric the challenges are huge for a man called Barack Hussein Obama.

The man who once walked on water has been limping of late, his feet weighed down by a whole catalogue of sinking issues - race, elitism, an excess of academic aloofness in times of gritty gravity.

Senator Obama has lost eight of the past 15 primaries.

He has been defeated in some places by devastating margins - he was trounced two to one by Hillary Clinton in West Virginia and Puerto Rico.

Mr Obama also lost Ohio and Pennsylvania, two important swing states he needs to win in November.

He has failed to impress Hispanic voters, millions of whom are expected to flock to the polls for the first time this year in other crucial swing states like Colorado, New Mexico and Nevada.

He has consistently lost support among white, blue-collar workers who may have every reason to loathe the Republicans but have yet to discover a reason why they should like Mr Obama.

Exotic mystery

I was recently in Michigan, an important state with a tanking economy.

The controversy over Mr Obama's former pastor has been damaging

The refrain I heard over and over again was that voters knew little about Barack Obama, apart from the fact that he had a wacky, inflammatory pastor.

Barack Hussein Obama remains an exotic mystery to most Americans.

He is trapped in the complex abstractions of his multi-cultural upbringing.

For millions of Americans, who have not read his book, who do not read the New York Times and who do not follow politics obsessively on the cable TV networks, he is a blank canvas, which his opponents can paint on.

That is dangerous and that is why one recent opinion poll suggested 15% of Americans still believe - astonishingly - that he is a Muslim, despite the problems caused by his former - Christian - pastor.

That is why in a recent RealClearPolitics poll of polls, Obama tied with Republican John McCain at 46%.

Independent voters

This should be the Democrats' year.

The economy is in serious decline. More than 80% of Americans believe the country is heading in the wrong direction, according to a poll last month.

The majority of Americans would like to see US forces leave Iraq

A slightly smaller number believe that President George W Bush has done a dreadful job.

His ratings hark back to the dank, dark popularity dungeon of the Watergate era.

Iraq continues to bleed soldiers, money and confidence.

The majority of Americans believe that the war was wrong.

They agree with Mr Obama that US troops should be withdrawn and disagree with Senator McCain that America should maintain a big military presence for the foreseeable future.

And yet they also think that the 71-year-old war-horse from Arizona is better equipped to execute a policy of withdrawal, even though the ageing senator does not believe in an early exit.

The poll number that should worry Obama most, however, is that among independent voters, the ones that actually swing elections, his favourability rating has gone from 62% in late February to 49% in May, according to the Pew Research Center.

In other words, a dwindling number of Americans think that Mr Obama is cool and clever and embodies the kind of change and hope that buoyed him in the polls at the beginning of the year.

'Nothing aloof'

So, with five months behind us and five months ahead of us, Barack Obama has his work cut out if he wants to become the country's 44th president and the first African-American ever to sit in the Oval Office.

Barack Obama (far right) did not come from a privileged background

In order to beat John McCain, he needs to rediscover the rhetorical mojo that got him this far and tell people who he really is, so that a few video clips from his former pastor, Jeremiah Wright, do not become his calling card.

There is nothing aloof about Barack Obama.

Although he went to Harvard his elite education was far less inevitable than that of Mr McCain, the son of an admiral who followed his father to the US Naval Academy in Annapolis and is married to a woman reportedly worth $250m (£128m).

Instead of migrating to a Wall Street law firm like most of his fellow graduates, Mr Obama went to the South Side of Chicago and did grassroots political organising.

This is not what elitists do.

The senator from Illinois won millions of supporters with his clever message that this election is about "you" and not about "me".

But it is also about "him".

He needs to tell his story in a clear way and back it up with a few crystal clear policy initiatives that make sense to ordinary voters.

[Mr Obama] needs to remind his audience that the Republicans are bitterly divided over all the big questions facing America today

He needs to feel the nation's pain.

As we know from his books and his campaign performances so far, he is a master storyteller.

On the policy front, he needs to remind his audience that the Republicans are bitterly divided over all the big questions facing America today.

What kind of superpower does the US want to be?

Feared or respected? Hunkering down at home or throwing its weight around overseas?

Barack Obama needs to find answers to these questions.

If there is one thing we have learned in recent years, it is that American voters are prepared to listen, they realise their country is at a vital crossroads and they value a candidate who feels comfortable in his own skin.

Heal the wounds

One marathon has just ended.

The other, even more arduous one is already about to start.

Mr Obama has effectively won the nomination on a losing streak. Mrs Clinton has lost on a winning streak.

The Democrats must unite their party if they are to defeat John McCain

He needs her to help heal the wounds of the Democratic Party and therefore his sworn enemy to become his most ardent campaign general.

Given all that has passed between them, it may be too much to ask for. Hillary may well adopt a work-to-rule attitude.

Perhaps she is even hoping that he will lose, which might mean that she could run against a 76-year-old President McCain in 2012.

His biggest strength remains the fact that he has fanned a burning desire for change among millions of young and black voters who never bothered with the polls before.

More than any other candidate, he has used the internet to enlist their money and their time.

He has raised many millions more than John McCain.

And he has already tried to paint his opponent as representing another term of George W Bush.

One should also remember the unknowns - another terrorist attack, which would help the old warhorse McCain, or a worsening war in Iraq, which would harm him.

At this stage in the race I would venture the following simplification - if America votes with its heart, it will elect Obama.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.